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SEMI E30-1103 © SEMI 1992, 2003 87 Figure A.5.3 Example of AND Substates A simplification that also helps to prev ent in determinacy is implemented with the symbol for default entry point. This symbol will indicate which…

SEMI E30-1103 © SEMI 1992, 2003

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A.5 Harel Notation

Harel’s statecharts extend traditional state-transition

diagrams with several additional concepts, most

important of which are hierarchy and concurrence.

Statecharts depict the behavior of a system by showing

states it may take, events that prompt a change of state,

and the composition of states. What follows is a very

brief description of the symbols defined for use and

how these are useful to describe a system. See Figure

A.5.1 for the basic notational symbols.

Figure A.5.1

Harel Statechart Symbols

States are represented by rounded boxes. A state

transition is shown graphically with a line from the old

state terminating with the arrow symbol at the new

state. Transitions are unidirectional-while the reverse

transition may be possible, it is considered a different

transition with different conditions for initiation and

different resultant actions.

States may be subdivided into substates to facilitate

more concise definition of behavior. Thus, a hierarchy

is defined whereby any state may be a substate of some

parent state and in turn be the parent of its own

substates. Substates must be one of two types, termed

AND substates and OR substates.

A parent maybe divided into two or more OR substates

of which one and only one is the active substate at any

time. The accepted term for this exclusivity is XOR.

Figure A.5.2 gives an example of a simple case of OR

substates. In this example, some system (perhaps a

motor) has a state named FUNCTIONAL. When the

motor is FUNCTIONAL, it may be either ON or OFF,

but never both.

Figure A.5.2

Example of OR Substates

Another way of dividing a parent state corresponds

roughly to subsystems. These AND substates represent

parallelism, such that every AND substate of an active

parent state is considered active. Harel also uses the

term “Orthogonal Component” to refer to AND

substates. However, these parallel substates tend to be

highly interactive and interdependent. For this reason,

the word orthogonal is considered confusing and has

been excluded from use in this document. Figure A.5.3

shows an example of AND substates representing (in

part) an automobile. Note the convention of attaching

the name of the parent state AUTOMOBILE to the

outside of the state in a small box. The substates shown

are independent components and may have their own

substates (of either the AND or OR type):

— LIGHTS may be ON or OFF;

— DOOR may be OPEN or CLOSED;

— ENGINE is constructed of components such as

pumps, pistons, carburetor, etc.

Exiting one of a set of AND substates requires the exit

of all others. In some cases, a transition arrow will be

shown from only one of the substates with the others

implied.

SEMI E30-1103 © SEMI 1992, 2003

Figure A.5.3

Example of AND Substates

A simplification that also helps to prevent in

determinacy is implemented with the symbol for default

entry point. This symbol will indicate which OR

substate is initially active when there is not an explicit

choice. This lack of specification is indicated by a

transition arrow from one state to another that does not

cross the boundary of the parent to point specifically to

a substate.

An entrance to a state terminating in a history symbol

(see Figure A.5.1) indicates that the OR substate to be

entered should be that which was active the last time

the parent state was active (i.e., last time the car was

running, the radio was on). The history symbol H refers

to the choice of substates of the parent. The symbol H*

extends further to the lowest level substates defined. In

the absence of memory of a “last time”, the default

entry is used.

The selector and conditional selector symbols serve to

abbreviate complex entrances to states. Their meaning

is similar and indicate that the choice of OR substate

upon entry of a parent state depends on some condition

that is not shown. The selector is usually used to

combine several similar transition events, while the

conditional selector will typically require some

computation or test of conditions external to the

stimulus for state transition. Please examine the

referenced article for more detail.

NOTE: Within the body of this document, the

term statechart is not used in favor of the more

traditional term state diagram.

A5.1 State Definitions –– The state diagram provides a

concise description of the function of a system.

However, a full definition requires detail that cannot be

included on the diagram. A description of each state is

required that covers the boundaries of the state and any

responses that occur within that state to the

environment. The convention in this document is to

provide state names in ALL CAPS to help the reader

identify where these are used. A sample state

description of the ON state depicted in the Figure A.5.2

might be:

The switch is in the on position. Power is available to

the motor. Speed of the motor will change in proportion

to the speed knob adjustment.

A5.2 Transition Table –– The last piece of the state

model is the transition table. It consists of several

columns that list the transition number from the

diagram, the starting and ending state for the transition,

and three columns titled trigger, action, and comment.

The trigger column describes the combination of events

and conditions that initiates the transition (e.g., message

Sx,Fy received). The trigger should be related to a

single clearly defined event at the equipment. The

action column identifies the activities associated

directly with the transition. These activities may be of

three types: a) actions taken upon exit of the old state,

b) actions taken upon entry to the new state, and c)

actions not associated with either state. These are not

differentiated in this document. The final column

allows for additional comments that help to clarify the

transition. Table A.5, an example of transition table,

illustrates the motor example in Figure A.5.2.

Table A.5 Transition Table for Motor Example

# Current

State

Trigger New

State

Action Comment

1 OFF Switch turned

to on position.

ON Power

supplied to

motor.

Power

supply

assumed

available.

Motor begins

to turn.

2 ON Switch turned

to off position.

OFF Power

supply to

motor dis-

connected.

Motor begins

deceleration.

A.6 Example Control Model Application

This section provides one example of a host’s

interaction with an equipment’s control model. A host

system must have a view of the control model to

understand and predict equipment behavior. However,

the implementor may simplify the host’s view by

assuming that some configuration settings are fixed and

that the host-initiated features are not implemented.

Applying these assumptions simplifies the behavior the

host expects to see.

Figure A.6.1 shows the effective control model

based

on the following host assumptions:

— The fundamental requirements are met, but the

additional host-intiated control capability is not

implemented.

— The configuration for the default entry to

CONTROL is set to an OFF-LINE substate (either

ATTEMPT ON-LINE or EQUIPMENT OFF-

LINE).

— The destination state for transition 4 (failure of

S1,F1 transaction) is configured to EQUIPMENT

OFF-LINE.

Figure A.6.1

Example of the Simplified “Effective” Control

Model

This view of the model has two further settings that the

host recognizes

as changeable at the equipment. The

first is the configuration of which substate of OFF-

24 See Section 3.3 for details of the control model.

LINE to be activated upon system initialization. The

second is the front panel switch that determines

whether the active system substate is LOCAL or

REMOTE when ON-LINE.

This application has the following implications:

— This application requires that the equipment begin

with the OFF-LINE state active. Thus, an

equipment initiated S1,F1/F2 transaction must be

completed before the equipment will begin sending

all messages to the host.

— If a failed attempt to go ON-LINE is made by the

equipment, it will not allow the host to complete

the transition at a later time. An operator will be

required to re-initiate the transition to ON-LINE

when the host becomes ready.

— Once ON-LINE, the equipment will remain ON-

LINE until an operator sets the equipment OFF-

LINE at the equipment front panel.

— Since all transitions into the HOST OFF-LINE

state are eliminated, this state is effectively

eliminated from the host view of the control model.

This application retains the following features:

— The ON-LINE state is achieved only after the host

acknowledges the equipment by replying to the

S1,F1 with and S1,F2. This confirms to the

operator attempting to put the equipment ON-LINE

that the host application is ready for work to begin.

— It provides the operator the means to set the

equipment OFF-LINE for non-host-related

activities

(e.g., maintenance, test lots).

— The operator has the ability to operate the

equipment with either the REMOTE or LOCAL

state active. As the equipment transitions to ON-

LINE, the preferred substate is automatically

chosen (based on a front panel switch).

— The user may configure which substate of OFF-

LINE the equipment will initially activate at

system initialization. If ATTEMPT ON-LINE is

chosen, the equipment will automatically attempt

the transition to the ON-LINE state as system

initialization.

25 Which activities are “non-host-related” varies from factory to

factory. In general, fewer activities are “non-host-related” as a

factory’s automation level increases.